Process of preparing a silica sol



Patented Mar. 2, 1954 UNITED STATES PATENT OF F ICEv PROCESS OF PREPARING A SILICA SOL Frederick J. Wolter, Wilmington, Dell, assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation; of Delaware N Drawing. Application January 5, 1952, Serial No. 265,195

8 Claims. I

This invention is directed to the preparation of silica sols by passing dilute solutions of lowrati'o alkali-metal silicates through the ammonium form of a cation-exchanger. The sols thus produced are stabilized by the addition of one portion of the sol to another portion heated to a temperature of 60 C.

It has, heretofore been proposed as in Bird 2,244,325 to prepare silica sols by passing alkalimetal silicate solutions through the acid form of a cation-exchanger. The sols thus prepared are very likley to gel. This is particularly true if sodium is removed to such an extent that the pH drops to around to- 6. The difiiculty is es,- pecially great at silica concentrations greater than about 3 per cent.

It has also been proposed to use the ammonium formof cation-exchangers for removing the alkali metal from alkali-metal silicates as in Voorhees 2,457,971. In such processes it was proposed to, use silica solutions containing more than per cent SiOz. While this can. be done with silicates which have a very high ratio of S102 to N320 such as are prepared by firsttreating the silicate with the acid form of a resin they cannot be used practically for ordinary silicates of low ratio. With alkalimetal silicates of low ratio, the deposition of silica on the resin is so excessive as to make the process impossible of commercial accomplishment. The products, moreover, which are made in this way from ordinary alkali metal silicates containing more than 10 per cent SiOz are extremely subject to gelling.

According to the present invention, the ammonium form of a cation-exchanger is used to remove the alkali metal of a low-ratio alkalimetal silicate in a solution containing less than 7 per cent $192. If heated, the silica sols produced are stable only as long as ammonia is'present but they may be stabilized by the addition of a strong base or by adding one portion of the sol to another portion, heated to a temperature above about 60 C. The exhausted exchanger may be regenerated by treatment with an ammonium compound.

The alkali metal silicate, such as sodium or potassium silicate, used to prepare sols according to the invention have a mol ratio of SiOzIMzO of from 1:1 to 3.9:1. tion which can be used can be expressed by the empirical relationship: 1

The maximum concentra- 2 Where R is the ratioof SiOzzMzO. of course, be expressed as 216R A solution of a 3.25 ratio silicate accordingly could be used with a maximum of 7' per cent of SiOz by weight. The solution can contain any smaller amount of silica though it is usually less economic to use smaller amounts than say about 1 per cent $102. The advantages of the invention are chiefly realized when the: solution contains such quantity of sodium silicate as amounts to more than 4 per cent SiOz by weight.

According to processes of the invention the sodium silicate solution is passed through the ammonium form of a cation-exchanger. Ordinariiy; the cation-exchanger is infinely divided form to permit intimate contact between. the solution and the exchanger. The mode of use of cation-exchangers is generally well, understood and is describde, for instance, in the Bird patent previously mentioned and in Hurd 2,431,481. The literature also makes: many references to suitable forms of equipment and to processes for effecting contact between solutions and cation-exchangers;

Any insoluble cation-exchanger may be used in: processes of the invention and there may be used, for instance, sulfonated carbonaceous exchangers or sulfonated or sulfited insoluble phenol-formaldehyde resins or acid-treated humio material, or other similar exchangers. Sulfonated coal, lignin, peat, or other insoluble sulfonated humic organic material may be used.

Even better are the insoluble resins made from phenols, such as those made from phenol itself, diphenyloi sulfcne, catechol, or naturally occurring. phenols, as found, for example, quebracho, and an aldehyde, particularly formaldehyde, which are modified by the introduction of sulfonic groups: either in the ring or on methylene groups.

The exchanger is generally prepared in a, grail u-lar form which is readily leached free of soluble salts. After the exchanger is exhausted by use, it may readily be converted to the am:- znonium form by washing with a suitable ammonium compound. such as ammonium chloride, ammonium sulfate, ammonium sulfamic, or ammonium nitrate. These materials may be used at a; slightiy basic pI-I, for example by the. addition of a small amount of an ammonium compound which is basic such as ammonium hydroxide or carbonate.

One of the preferred cation-exchangeresins for use according to the present invention. is: an aromatic hydrocarbon polymer containing, nuclear sulfonic: acid groups which is designated This can 3 "Dowex 50 and of the general type described in DAlelio 2,366,007 and which is fully described as to its characteristics, properties, and general mode of use in the Journal of the American Chemical Society for November 1947, volume 69, No. 11, beginning at page 2830.

The effluent from the cation-exchanger will contain ammonia corresponding to the sodium removed. If an effort is made to concentrate the efiluent simply by boiling, the ammonia will be driven off and the sol will gel.

If the sol is to be concentrated by evaporation it should be stabilized by the addition of a strong base. The base should be used in such an amount as to make the pH of the sol fall within the range from about 8 to 10.7.

As suitable bases there may be mentioned sodium hydroxide, sodium silicate, potassium hydroxide, and potassium silicate. If an alkali metal M, such as sodium or potassium, is used as the base, the final molar ratio SiO2:M2O should be above about 25:1. It is preferred that the ratio be from about 60:1 to 120:1. After the effluent has been stabilized by the addition of a base as indicated, it can then be concentrated by evaporation. Concentration can be carried to about, say, 10 per cent SiOa by weight, or even a little higher.

The ammonia which is evolved from the sol can be recovered and can be used to regenerate the cation-exchanger. This can be done by absorbing the ammonia in dilute sulfuric acid to obtain a solution of ammonium sulfate and by passing this through the exchanger. A small excess of ammonia can be present so that the solution will be slightly on the basic side.

Another method which may be used is to regenerate the exchanger with sulfuric acid to get it in the hydrogen form. Then the ammonia given off during the concentration is absorbed in water and passed through the hydrogen form of the cation-exchanger to convert it to the ammonium form. The ammonia gas which is evolved can be absorbed in a water slurry of the exchanger.

It should be observed in regenerating the exchanger that if the hydrogen form is used it must be converted substantially completely to the ammonium form since any significant amount of the hydrogen form of the resin will cause gelling in the resin bed.

More stable products can be made by heating one portion, say, half of the efiluent with base added, to a temperature above 60 C. and slowly adding the remainder to it while continuing the heating. Preferably, an even smaller portion of the efiluent, say, about a fifth, is first heated and then the remaining portion slowly added over a period of several hours while continuing the heating to a temperature above 60 C. This procedure is described and claimed in U. S. Patent 2,574,902, dated November 13, 1951, of Max F. Bechtold and Omar E. Snyder.

The sols can be concentrated by evaporating some of the water during the heating steps just described.

Processes such as those described can be carried out by heating a portion of the efiluent to which no base has been added and then adding further quantities of eflluent at such a rate as to compensate for loss of ammonia. The pH should not drop to below about 7.5 and it is better to maintain the pH above 8. A pH between about 8 and 9 would be desirable for this method of operation.

The sols prepared can be concentrated to 30 or up to 35 per cent S102. Even higher concentrations, say up to 50 per cent SiOg can be obtained by deionizing the sols with anionand cation-exchangers and then adding back a little alkali before concentration.

However, as has already been indicated, it is best to add a strong base which is not volatile so that such rigid control need not be exercised. The pH of the sol with a strong base can be adjusted to pH 8 to 10.7.

In order that the invention may be better understood reference should be had to the following illustrative examples:

Example 1 A sodium silicate solution of 3.25 Si0z:NazO mol ratio is made up containing 6 per cent $102 by weight. This solution is passed through the ammonium form of Dowex 50 resin. The effluent contains approximately 6 per cent $102 by weight and contains substantially no sodium. The resin is regenerated by washing with ammonium chloride containing a little ammonium hydroxide and used for a later batch.

A small amount of sodium silicate is added to the efliuent to bring the ratio of SiOzzNazO to :1. The solution is then boiled to concentrate it by evaporation. Ammonia is lost and the solution is concentrated to 10 per cent SiOz. The sol thus obtained is practically clear and is stable for some months.

Example 2 An efiluent is prepared as in Example 1 and 1 6 of the total efiiuent is placed in a receptacle and heated to 0., just below the boiling point. The remainder of efiluent is slowly added over a period of ten hours. During this time a considerable portion of the ammonia is lost but the solution still remains at a pH above about 8.5. The sol has an S102 content of about 6 per cent and this is then concentrated by evaporation to 30 per cent SiOz. During the evaporation a lit tle ammonia gas is added throughout the evaporation to keep the pH above about 8.

Example 3 A procedure is followed as in Example 2 but prior to the heating of the efiluent, sodium silicate is added in an amount to give an SiOzzNazO ratio of 9011. Thereafter, of the solution is placed in a receptacle, heated to boiling, and the remaining /5 is slowly and steadily added over a period of five hours during which time water is evaporated to maintain essentially a constant volume. The resulting sol is quite stable and has a molecular weight of around 2,000,000. The SiOz content by weight is 30 per cent.

This application is a continuation-in-part of my co-pending U. S. application, Serial No. 168,143, filed June 14, 1950, for Process, now abandoned.

Iclaim:

1. In a process for making a silica sol the step comprising passing through the ammonium form of a cation-exchanger an aqueous solution of an alkali-metal silicate of SiOz:M2O ratio from 1:1 to 3.9:1 and having an SiO2 content less than per cent where R is said ratio.

2. In a process for making a silica sol the step comprising passing through the ammonium form of a cation-exchanger an aqueous solution of an alkali-metal silicate of SiOzzMzO ratio from 1:1 to 3.9:1 and having an SiOz content less than per cent where R is said ratio, separating the effluent solution from the exchanger, heating a portion of the effluent solution to a temperature above 60 C., and adding another portion of the effluent solution while continuing to heat, the pH being maintained during said heating above 7.5 by use of a base selected from the group consisting of ammonia and alkali metal bases.

4. In a process for making a silica sol the step comprising passing through the ammonium form of a cation-exchanger an aqueous solution of sodium silicate of SiOzZNazO ratio from 1:1 to 3.9:1 and having an SiOz content less than per cent wher R is said ratio, separating the eflluent solution from the exchanger, heating a per cent wher R is said ratio, separating the solution from the exchanger, adding to the efliuent solution a sodium compound from the group consisting of sodium hydroxide and sodium silicate in an amount to yield a pH above 7.5 and to give a ratio of sodium to silica such that the SiO2:Na2O ratio is above 25:1, heating a portion of the eflluent solution to a temperature above C. and adding to this portion another portion of the eflluent solution which is at least 4 times as large while heating above 60 C. and concentrating the sol by evaporation to an SiOz content of at least 10 per cent.

6. In a cyclic process for making a silica sol the step comprising passing through the ammonium form of a cation-exchanger an aqueous solution of an alkali-metal silicate of SiOzZMzO ratio from 1:1 to 3.921 and having an SiOz content less than per cent where R is said ratio, separating the solution from the exchanger, adding an alkali metal base to the solution in amount to yield a pH above 7.5, removing ammonia, regenerating the cation-exchanger to the ammonium form with said ammonia, and returning said cationexchanger to the process for treatment of a further quantity of silicate.

7. In a process for making a silica sol by Withdrawing alkali metal from a soluble alkali metal silicate, the steps comprising contacting the ammonium form of a cation-exchanger with an aqueous solution of an alkali-metal silicate of SiO2:M2O ratio from 1:1 to 3.9:1 and having an S102 content less than per cent where R is said ratio, and continuing such contact until enough alkali metal has been removed to raise the SiOz:M2O ratio above 25.1.

8. In a process for making a silica sol by wit drawing sodium from a soluble sodium silicate, the steps comprising contacting the ammonium form of a cation-exchanger with an aqueous solution of sodium silicate of SiO2:MazO ratio below 3.9:1 and having an S102 content of at least 4% but less than per cent where R is said ratio, and continuing such contact until enough sodium has been removed to raise the SiO2:Ma2O ratio above 60:1. FREDERICK J. WOLTER.

References Cited in the file of this patent UNITED STATES PATENTS Name Date Voorhees Jan. 4, 1949 Number 

3. IN A POROCESS FOR MAKING A SILICA SOL THE STEP COMPRISING PASSING THROUGH THE AMMONIUM FROM OF A CATION-EXCHANGER AN AQUEOUS SOLUTION OF SODIUM SILICATE OF SIO2:NA2O RATIO FROM 1:1 TO 3.9:1 AND HAVING AN SIO2 CONTENT LESS THAN 